Metal detector



Dec. 21, 1943.

C. B. SITTERSON, JR. El"AL METAL DETECTOR Filed Aug. 24, 1942 2 Sheets-Sheet l Fig.7.

Inventor's: Cephas B. Sitterson Jrz, Theodore H. Berry, b MW, a. swim 'T'he'lr AttoPrie Dec. 21, 1943.

C. B. SITTERSON, JRv EI'AL METAL DETECTOR Filed Aug. 24, 1942 2 Sheets-Sheet 2 TOISIGNAL AMPLIFIER Fig. I0.

FIBER Fig. l2.

I as

Inventors: 7 Cephas B. Sitterson- Jn,

Theodore M. Berry, IRON b 'fiwy j w.

Them Attorney.

Patented use. 21, 1943 DETECTOR Cephas B. Sitierson, In, and Theodore M. Berry,

Schenectady, N. Y'.,

minors to General lilac-'- tric.Company, a corporation of New York Application August 24, lllitzgsserlal No. 455,844

2 Claims. (0. 115-183) Our invention relates to apparatus for detecting the presence of magnetic and non-magnetic metal parts otherwise hidden in non-magnetic non-conductive material. Forexample, the invention may be used to detectnails and screws in lumber. An important object of our invention is to detect nails or other metal objects in lumber and the like, regardless of how the hidden nail may be oriented and passed past the detecting apparatus.

Our invention finds application in the woodworking industry to detect nails and the like in lumber soas to'avoid dulling or damaging the wood-working tools by the presence of nails and the like in the lumber used. In illustratingour invention we provide inspection apparatus consisting of exciting and detecting coils between which lumber in the shape of boards, planks and view of the lower coil section of Fig. 10 as seen from the left end of Fig. 10.

.Referring now to Fig. 1, l and II represent I exciting coils which are excited in seriesfrom an I alternating current source ll. Spaced from coil I0 is a detecting coil l5. Coils I4 and it are coaxial, are hollow as represented and are separatedsufllciently that the lumber to be inspected tion so that the voltages induced therein by their the like may be passed for the purpose of detecting nails and the like hidden in the lumber and giving a warning indication thereof and the approximate location oi the hidden nail.

Apparatus of this general character is not new but difllculty was experienced with apparatus heretofore available for this purpose because of its inability to detect all nails, regardless of their location and orientation in the lumber and our invention relates to improved apparatus in this respect.

The features of our invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of our invention reference is made in the following description to the accompanying drawings in which Fig. 1 shows simplified detecting coil apparatus to be referred to in explaining'the principles of our invention, Fig. 2 illustrates the normal flux distribution of the upper set of coils of Fig. l when no nail is present, Fig. 3 illustrates the flux distribution due to the presence of a magnetic nail oriented as shown, Figs. 4, 5 and 6 illustrate various relative orientations of a magnetic nail with respect to the detecting coil axis, Fig. 7 illustrates the flux distribution of a coil set such as the upper coils of Fig. 1 for the nail orientation represented in Fig. 4, Fig. 8 represents such flux distribution for orientation represented in Figs. 5 and 6, Fig. 9 illustrates in principle the staggering of the detecting coils and their alternate use as detecting and balancing coils, Fig. 10 is a perspective view of a preferred embodiment of our invention for inspecting planks, Fig. 11 shows a wiring diagram of the detecting coils of Fig. 10 and Fig. 12 is a 1 axis as represented in Fig. l.

exciting coils II and II oppose each other and when the flux distribution is not influenced by the presence of magnetic or conductor material the secondary voltages are adjusted so as to be equal. As thus described. coils II and I act a a, balancingcoil set for the detecting set of coils. Under such conditions the flux distribution of either set of exciting and balancing coils may be generally represented as in Fig. 2 where some of the flux d of the exciting coil lothreads the detecting coil l5 and some flux L doesnot. For convenience we will refer to the flux d which threads the detecting coil as detecting flux and the flux L which-does not thread the detecting coil as leakage flux; The two sets of coils may be similar and alternately used as detecting and balancing coil sets.

In the circuit of secondary detecting coil II and its balancing coil is is a suitable voltage responsive device I! which produces a response when the opposing secondary voltages do not balance. If, now, a nail 20 be passed between one set of exciting and detecting coils, we have found that the flux distribution of such set may be changed and the nature of the change will depend upon the. relative position of the nail with respect to the adds of the coils and the orientation of its longitudinal axis with respect to the flux axis of the coils. For example, let us assume that a piece of lumber containing an iron nail 20 be Passed between coils I ii and it in the direction represented by th arrow adjacent the nail and that the nailpasses centrally between the coils with its longitudinal axis parallel to the flux This will increase the detecting flux relative to the leakage flux and we may represent this changed flux distribution as in Fig. 3. Under these conditions it is evident that coils II and I6 will retain the relative flux distribution represented in Fig. 2 but that coils Ill and I5 will momentarily have the flux distribution represented in Fig. 3. Hence the secondary voltages will'become unbalanced and distribution due to the presence oi. the nail or at least no change that produces a definite response of. instrument It. Apparently the flux distribution remains essentially the same as in Fig. 2, as shown in Fig. "I. If, however, the nail be vertical and with its longitudinal axis perpendicular to the flux axis but passed horizontally between the coils near their upper or lower edges, as represented in Figs. and 8, we have found that the normal flux distribution is disturbed and that the nail materially increases the leakage flux and decreases the indicating flux, as represented in Fig. 8. This also produces'a responsive indication at IS. The same sort of flux disturbance is produced if thenail be passed horizontally between the centers of the coils with its axis horizontal and perpendicular to the flux axis, as represented in Fig. 6. Fig. 8 may represent the flux distribution resulting from the position of Fig. 6 if we consider the nail as now moving downward in Fig. 8. As thus considered, as the nail reaches the center of the coils the flux distribution will change to that represented in Fig. 7 and as the nail passes downward between the lower edges of the coils, the flux distribution will change again to that represented in Fig. 8, except that the leakage and detecting fluxes will change places vertically. Hence, there will be a double response of the indicating apparatus I9, once as the nail-enters and once when it leaves the coils. We have thus illus trated some of the most sensitive and least sensitive detecting conditions and there are a variety of others of an intermediate sensitivity. For

- example, if for the condition represented in Figs.

1 and 3 the nail be oriented through an angle of 45 degrees on an axis perpendicular to the drawing as represented in dotted lines in Fig. 3,

the condition will become less sensitive but the.

nail will still have considerable flux directing force tending to increase the detecting flux and, by providing a sensitive indicator, its presence will be detected.

Now, in order to detect nails and the like in a board or plank for those conditions such as-represented in Figs. 4 and '7, for example, we provide a now, a nail 20 be passed along a horizontal line to the right as indicated by the arrow, it will pass the peripheral edge of coil l I. While the exciting coils l0 and II are not shown, it is assumed that they are axially behind coils l5 and I0 and that the nail-carrying lumber passes between the exciting and detecting coils. When the nail arrives opposite coil IE, it will produce little if any disturbance of the field, but when it arrives opposite coil l 6 in the position indicated in dotted lines, it will produce a disturbance of the field.

analogous to that shown in Fig. 8'and a response of indicator l9 will result. If another nail 20a, oriented as shown, follows nail 20 in the path indicated, it will produce an indication when passing coil I! but not when passing coil It. The

staggered multiple coil arrangement of our invention is thus illustrated. Furthermore, it will be noted that while both coils are used as detecting coils, they are also both used as balancing coils for each other.

It should be noted also that while the condition represented in Fig. 3 produces an increase in flux in the detecting coil l5 and the condition represented in Fig. 8 produces a decrease in flux in the detecting coil I5, both conditions. produce an indication because both produce flux conditions such as steel or iron nails.

in which thereexists non-magnetic conductor material. For example, if the nail 20, Fig. 7, were ofbrass instead of being magnetic and a high frequency flux of the order of 50,000 cycles were used, the nail would tend to form an eddy current shield between the two coils, decreasing the passage of flux to the detecting coil l5 and the detecting flux would decrease from a normal condition, thereby detecting the presence of the plurality of groupsof exciting and detecting coils which are so staggered in overlapping relation with respect to the path of travel of the lumber which is passed therethrough that if a nail is not detected by one pair of coils it will be by a second or subsequent pair.

In Fig. l the pair of coils I I and I6 were merely described as balancing coils for the pair |0--|5. However, when a number of sets of coils are arranged in staggered relation along the path of travel of the lumber, we connect them in groups as in Fig. 1 so that all serve alternately both as detecting and balancing pairs of coils, as illustrated in Fig. 9 and in a practicable example in Figs. 10 and 11.

In Fig. 9 we have shown the detecting coils l5 and I8 of Fig. 1 spaced apart in the line of travel of the lumber to be inspected, which is assumed to be in a horizontal direction to the right. Also, coil I6 is elevated with respect to coil l5. If,

ill

metal. The action of the detecting apparatus with respect to non-magnetic metals is in general the reverse of the action with respect to magnetic metals when high frequency excitation is used and both magnetic and non-magnetic metal parts will be detected simultaneously.

Fig. 10 represents apparatus which is employed to detect metal parts in lumber used for making gun stocks. In this illustration 2| represents a suitable longitudinal guide frame made from fiber and wood parts through which the lumber 22 is passed to test the same for metal inserts. 23, 24,v 25 and 28 are the outer end supports of the test coils 21, 28, 29 and 30. The test coils are secured on one outer wall of the guide 2| as represented in Fig. 12 by being clamped between such wall and the support 23 by a non-magnetic bolt 3|. The corresponding exciting coils are axially displaced and supported'on the opposite outer wall of the guide, as shown at 32 in Fig; 12. It is noted that the lowermost set of exciting and detecting coils 33 and 21 have their centers 'in line with the bottom edge 34 of the opening in the guide while the upper set of coils including coil 30 have their centers on line with the upper edge 35 1 of the guide opening. While the other sets of coils including detecting coils 21 and 28 have intermediate positions the coils overlapping about half their diameter in the line of travel oi the lumber so as to detect nails and the like in the lumber 2! regardless of the position and orientation of the nails in the lumber as the same is slid through the guide during a testing operation. We have found that the sensitivity can be improved by providing thin magnetic plates 38 extending across the outer ends oi the detecting coils, as indicated.

Provision is made for exactly balancing the detecting coils both as regards amplitude and phase angle and connecting them in series opposition, as will be. explained in connection with Fig. 11. In Fig. 11 the four detecting coils 21, 2O, 29 and 30 are represented as enclosed by dotted line rectangles. Coils 21 and 30 include amplitude adjusting sections 31 and phase adjustin sections 38. Coils 21 and 28 are connected in series opposition to coils 28 and Ill with an amplifier 39. The exciting coils will be connected in series to a suitable source of A. 0. supply.

It is evident that the voltage induced in coil II for a given excitation flux can be varied by varying the relative resistance at 40 in the two parts of the amplitude adjusting coil section 31 thereof, the two parts of the coil section 21 being connected in opposition. Hence, the coils 21 and 28 may be balanced as regards voltages. Also, the phase angle of coil 21 may be adjusted by adjusting the relative impedance oi a resistor ll as compared to a parallel circuit through condenser 42 and coil section II. Thus the phase angle of coil 21 may be adjusted and made equal to that of coil 28. The series connection between coils 21 and 2a is through an adjustment along a resistance 43. In like manner coils 29 and 30 may be balanced with similar adjustments associated with coil 30 so that for lumber which is free from metal parts the detecting coil circuit will be balanced as to amplitude and phase angle. I! desired coils 21 and It might be contained in one detector circuit and coils 29 and III in another as in Fig. 1 but this is unnecessary. Now, when a nail, screw or the like is contained within the lumber 22 which is passed through the testing apparatus. the detecting circuit will become unbalanced one or more times, either due to an increased or decreased induced voltage in one or more of the coils. The resulting unbalanced voltage in the detecting circuit is amplified by the amplifier 39 which may be contained within the box ll of Fig. and a suitable indication or alarm given on the instrument 4! or alarm 40. After becoming iamiliar with the operation of the apparatus the approximate location of the metal particle generally can be ascertained and that part of the lumber containing the metal discarded. The adjustable resistance features may conveniently be mounted on the base of the test apparatus as indicated in Fig. 10.

What we claim as new and desire to secure by Letters Patent in the United States is:

1. In apparatus for detecting the presence of metal parts in material such as lumber, a pair 01' sets of exciting and detecting coils, the set of a pair being axially aligned and displaced so as to act as an air core transformer with considerable leakage and to allow the material to be tested to be passed between them, means for energizing the exciting coils with alternating current so as to induce voltages in the detecting coils, means for connecting the detecting coils in series opposition under conditions such that the induced voltages balance when the inducting flux-is uninfluenced by the presence of metal parts, means for detecting unbalance in the induced voltages, provisions for passing the material to be tested between the exciting and detecting coils of both sets such that a given point in the material which passes centrally between one set of coils passes oil-center with respect to the other set of coils, whereby both sets of coils are available for use alternatelyas detecting and balancing sets.

2. Apparatus for detecting the presence of metal parts in material such as lumber, comprising a longitudinal guideway through which the material to be tested is adapted to be passed, an even number oi pairs of exciting and detecting coils supported on said guide, the exciting coil and detecting coil of a pair being axially aligned and supported on opposite sides of the guideway with their axes at right angles to the path of travel of the material through the guideway such that the material may be passed between them, the different pairs 01! coils being displaced along the line of travel of the material and the coils of one pair overlappin the coils of another pair about hallway with respect to the line of travel of the material, such that a given point of the material which passes between the centers or one pair oicoils will pass between the peripheral edges or another pair of coils, means for energizing the exciting coils with alternating current so as to induce voltages in the detecting coils, means for connecting the detecting coils in series opposition in a detecting circuit including adjustable means ior balancing the opposed voltages when the induced voltages are uninfluenced by the presence of metal parts in the material being tested and voltage responsive means for detectin voltage unbalance in the detecting circuit.

CEPHAS B. JR. THEODORE M. BERRY. 

